Create tryDispatchMountItems runnable only when needed (#53196)

Summary:
Pull Request resolved: #53196

In the FabricUIManager, the runnable created for scheduled mounts is only used if currently running on the UI thread.

With this diff the runnable only gets created when needed.

Changelog: [Internal]

Reviewed By: christophpurrer

Differential Revision: D79969412

fbshipit-source-id: ee78890322af8580357389aad8357f7c0d18490f

Author: Nick Lefever

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/fabric/FabricUIManager.java

@@ -935,14 +935,14 @@ private void scheduleMountItem(
     if (shouldSchedule) {
       Assertions.assertNotNull(mountItem, "MountItem is null");
       mMountItemDispatcher.addMountItem(mountItem);
-      Runnable runnable =
-          new GuardedRunnable(mReactApplicationContext) {
-            @Override
-            public void runGuarded() {
-              mMountItemDispatcher.tryDispatchMountItems();
-            }
-          };
       if (UiThreadUtil.isOnUiThread()) {
+        Runnable runnable =
+            new GuardedRunnable(mReactApplicationContext) {
+              @Override
+              public void runGuarded() {
+                mMountItemDispatcher.tryDispatchMountItems();
+              }
+            };
         runnable.run();
       }
     }